The invention relates to a method of manufacturing optical fiber preforms, which method is more particularly intended for manufacturing preforms of large diameter.
A known method of manufacturing or building up, i.e. xe2x80x9covercladdingxe2x80x9d, preforms designed for the production of optical fibers makes provision to deposit silica on a primary preform which is equipped with supporting end-pieces at its ends, and which is carried by means enabling it to move along its axis and to rotate relative to a flame of an inductive plasma torch, into which silica grains are injected and in which they are melted. That method makes it possible to manufacture a preform of determined thickness, starting from a primary preform that is thinner, by superposing a succession of concentrically-deposited layers of silica. The respective lengths of the successive layers decrease so that the thickness of deposited silica that covers the preform and its end-pieces tapers uniformly towards the ends, from a central segment of determined length and diameter. A conical shape is imparted to one of the ends of each preform so as to facilitate the subsequent fiber-drawing operations starting from said end.
It is known that the desired conical shape at the fiber-drawing end of the preform can be obtained by drawing the-end-pieces of the preform as overclad with silica in opposite directions, after the preform has been locally melted by being heated to the core, in a zone of the central segment that is situated in the vicinity of the end to be made conical. Such drawing makes it possible to separate the preform from one of its two supporting end-pieces. The intense heating performed for drawing purposes suffers from the drawback of giving rise to high silica evaporation, the evaporated silica then cooling and forming soot which falls back in particular on the preform. Such soot affects the transparency of the preform and increases its roughness.
In order to remedy those drawbacks, the Applicant""s Document EP-A-0 831 070 makes provision to perform the separation in two steps. A first drawing step causes the diameter of the preform in the zone that is heated for cleaving purposes to be reduced to a chosen diameter that is usually close to the diameter of a supporting end-piece. A glazing operation is provided for removing the unwanted deposit that has formed on the cold portions of the preform that have received soot during the heating performed for the first drawing step. The glazing is performed by passing the preform through the flame of the plasma torch without any material being supplied. A second drawing step terminates the separation in the smaller-diameter zone that is heated to the core for that purpose. The quantities of silica evaporated and re-deposited are small because of the small dimensions of the zone that is then heated.
That method including two separation steps is quite suitable when the preforms to be cleaved are of diameter that is not too large, e.g. of maximum diameter of about 80 mm.
However, it is not suitable when the preform diameter is such that heating to the core is lengthy and difficult to perform, thus making it difficult to obtain sufficient softening of the preform in the separation zone in which the drawing-apart is to be performed.
In addition, prolonged heating of a large-diameter preform gives rise to considerable silica evaporation from the heated zone, and causes a thick layer to be re-deposited on the cold portions of the preform and in particular on the portion close to the region that is to constitute the cone. That thick re-deposited layer does not necessarily vitrify properly during the intermediate glazing operation, and can give rise to a non-homogeneous zone that could disturb fiber drawing.
To remedy those drawbacks, the invention provides a method of manufacturing or building up, i.e. xe2x80x9covercladdingxe2x80x9d, an optical fiber preform in an installation provided with means enabling a preform held horizontally at its ends between two mounting points by supporting end-pieces to be rotated axially and to be moved relatively in translation. Heater means for heating the preform by means of a plasma torch, which heater means are disposed radially relative to said. preform are associated with material supply means, so as to enable the preform to be manufactured in successive passes corresponding to the preform and the torch being displaced relative to each other, with or without material being supplied, these displacements therefore leading either to a new layer of material being deposited on the preform, or to the most recent layer deposited being glazed.
According to a characteristic of the invention, the method makes provision to interpose a one-ended reduction in the length of at least one layer, during a pass and starting from one of the intermediate layers, while a succession of concentric layers of material are being deposited on the preform in a manner such that the respective lengths of the layers, which lengths are determined by the preform/torch relative displacements, are progressively shortened as a result of a progressive reduction in the lengths of the displacements, so that the thickness of deposited material that covers the preform and a portion of each of the end-pieces decreases uniformly towards the ends, said one-ended reduction in layer length leading to a limitation of the thickness of material deposited on one of the end-pieces and on a limited-length preform zone that is longitudinally adjacent to said end-piece, at the level set by the layer deposited immediately prior to said one-ended reduction.
According to a characteristic of the invention, the method includes at least one hot drawing operation performed to separate a preform from one of the end-pieces in said limited-length preform zone which is adjacent to said end-piece, after said succession of layers required for forming the preform has been deposited.
According to a characteristic of a variant of the invention, the method includes a hot-drawing operation performed in two steps, separated by a preform glazing operation, in said limited-length preform zone which is adjacent to an end-piece so as to separate the preform and said end-piece, the first drawing step being associated with heating to the core causing softening by melting in said limited-length preform zone, and producing a reduction in diameter, the second step also being associated with heating to the core causing softening by melting and achieving full separation.